Re: How do intercoms work?

The basic principle behind intercoms, telephones and other gadgets that reproduce sound at a distance, is that mechanical energy can be converted into electrical energy and vice-versa. So, we can convert the sound waves into varying electric currents in a wire, that at the other end are converted back to sound.

The way this works can be demonstrated if you can find a strong horseshoe magnet and some thin solid core (not stranded) insulated wire. Household telephone wire is good, or ask for some 24 gauge solid wire at your electronics store. Take about 15 cm (6 in) of the wire, and strip a centimeter (.4 in) or so of insulation off each end. Make the bare wire into loops at both ends, and fold the wire at right angles at the one- third points, so that you have three sides of a square. Now you need to stand the magnet on its side so that the poles are one above the other; you might need some clay to prop it up. Hang the little wire "swing" from two more wires, also with their ends stripped of insulation, in such a way that the bottom of the swing is between the poles of the magnet, and you can connect a small battery (D cell is fine) to the ends of the support wires, thus passing an electric current through the swing. Don't leave the battery connected for more than a fraction of a second or the wires will get hot. If everything works as it should, the swing will either jump into the horshoe or out of it, depending which way you connect the battery.

What this shows is that a current flowing across a magnetic field produces a mechanical force. The opposite is also true; if you apply mechanical energy to move a wire across a magnetic field it creates a current if there is a circuit to carry it.

So, the sound (vibrations in the air) of the person talking into the intercom causes vibrations in a microphone. There are many types of microphone but the one that uses the principle just mentioned has a coil of wire surrounding a strong magnet. The moving coil generates a small current, which is amplified by an electronic circuit to make a bigger current, but one that has the same frequencies and patterns as the original current, and hence the same as the sound. This current is sent down the wires to the other intercom station, where the reverse process occurs. The current is fed into a speaker, which is also constructed with a coil of wire surrounding a magnet. The interaction of the current with the magnetic field makes the coil vibrate. The coil is fastened into the centre of a light (often paper) cone that then vibrates, moving the air and reproducing the original sound.

Here is a link to a web page on speakers: http://electronics.howstuffworks.com/speaker.htm A speaker can also be used as a microphone, because the electrical to mechanical energy process is reversible, but it wouldn't be very convenient so the design of microphones is generally different.

Hope this helps;
David Lay
Kingston, Ontario.